In recent years, the production of advanced materials, such as electronic materials requires particle size reduction and high purification, and mills used therefor is mainly agitator mill in which grinding and dispersion media are stirred at a high speed compared with prior ball mills and which have a high grinding and dispersing efficiency. Such mills use ceramics that are excellent in impact properties and wear resistance as a material for mills as the material is under considerably high load. In particular, Y2O3-toughened zirconia (Y-TZP) composed of tetragonal zirconia is used as materials for mill parts.
In the agitator mill, it is unavoidable to result in a rise in temperature in the mill in operation. Thus, materials for mill are under considerably high load as they are exposed to high temperature and a corrosive component such as water, and further stirred with grinding and dispersion media. Therefore, it becomes important that the materials have stable durability for a long period in mechanical properties such as strength and wear resistance.
On the other hand, as ceramics have excellent wear and corrosion resistance and are lightweight, compared with metal, they have become so as to be used as material for machine elements with wear resistance, in particular bearing balls (rolling elements) in bearings in place of the prior metals. The bearings in which bearing balls made of ceramics are used provide stable rotation compared with ones made of metals when the rotation rate of a hard disk is increased in order to improve recording density. Therefore, such bearings have become so as to be used in spindle motors for driving hard disks in recent years. For example, bearing balls made of silicon nitride are used in order to accelerate the rate of spindle motors and improve lubricant life under high temperatures.
However, as spindle motors are used in a state where preload is applied to ball bearings in order to attain high rotational precision and linear expansion coefficient of silicon nitride is lower than that of ball bearing steel used in ball bearings, the bearing balls made of silicon nitride have problems that the preload therein varies largely depending on a rise in temperature, and therefore the bearing balls can not provide high rotational precision and are liable to lead to errors in reading and writing.
Japanese Patent Laid-open No. 2000-74069 discloses that bearing balls made of zirconia having a linear expansion coefficient close to that of ball bearing steel can reduce changes in preload due to temperature variations and thus improve rotational precision at a high rotational rate. However, the publication merely specifies the linear expansion coefficient of the bearing balls made of zirconia, and does not specify mechanical properties and the like as a sintered body for forming excellent bearing balls at all.
In addition, it has been pointed out that zirconia based sintered bodies give rise to deterioration as mentioned below and the sintered bodies have problems that they are not fully suited for bearings that are required to be used stably under several environments for a long term.
Further, also in the field of electronic and communication equipments, zirconia ferrules used as connectors for optical fibers are required to have 100% of reliability in durability so as not to give rise to change in shape due to transformation even when they are used at a temperature of 50° C. or more for a long time.
Although Y2O3-toughened zirconia (Y-TZP) that is a zirconia based sintered body has excellent mechanical properties, it is known that Y-TZP gives rise to transformation from tetragonal zirconia to monoclinic zirconia at a specific temperature range from 200 to 300° C. and causes thermal degradation leading to a lowering of strength. Japanese Patent Publication No. Sho 61-21184 proposes many countermeasures therefor. But it has recently become clear that the above-mentioned zirconia sintered bodies are lowered in mechanical properties such as strength or wear resistance, or cause small changes in dimension when they are exposed to a high temperature of 60 to 150° C. at which the zirconia is used, or an aqueous medium or a high humidity atmosphere at the high temperature under particularly high load conditions for a long time. And, it has become clear that the countermeasures against thermal degradation at 200 to 300° C. can not resolve the above-mentioned problems.
A zirconia sintered body is improved in durability such as wear resistance under load at a relatively low temperature of about 20° C. as the crystal structure thereof becomes fine but lowered in resistance to corrosive component such as water. In addition, the prior zirconia sintered bodies are lowered in mechanical properties such as wear resistance under load at a high temperature of about 60 to 150° C. in a dry or a high humidity atmosphere or an aqueous medium. Thus, for example, the prior zirconia sintered bodies do not satisfy requirements as materials for bearings that are rotated at a high rate under load and cause a rise in temperature, and therefore are required to be further improved.
Japanese Patent Laid-open No. 2000-302548 proposes a method of producing a zirconia sintered body excellent in thermal stability by limiting the crystal grain diameter to a specific range and incorporating a third component including Al2O3, SiO2 and TiO2.
However, even when the third component including Al2O3, SiO2 and TiO2 is incorporated, glass phases are formed in the zirconia sintered body due to impurities such as an alkali metal or an alkaline earth metal present therein and the glass phases may lead to a lowering in durability. Thus, the zirconia based sintered bodies can not be fully improved only by adding the third component mentioned above. In addition, the method disclosed in the Patent publication can prevent thermal degradation of zirconia based sintered bodies in a case where it is exposed to a warm water or a high humidity atmosphere near 200° C., but has not attained a perfect prevention of lowering in durability in a case where the product is particularly under load at a practical temperature range of about 60 to 150° C.
In addition, the crystal grain boundary of the zirconia sintered body becomes weak when it is exposed to load under a condition without humidity or steam at a temperature of about 60 to 150° C. As a result of it, the product has also a problem to cause a rapid wear due to loss of crystal grains.
Consequently, it has been required to provide materials that have excellent and thermal stability, corrosion resistance, mechanical properties and durability at an elevated temperature of about 60 to 150° C. in a dry or a high humidity atmosphere, or in an aqueous medium over a long period and that have excellent strength and wear resistance regardless of whether a high humidity atmosphere is present or not.
An object of the present invention is to provide a zirconia based sintered body exhibiting excellent thermal stability and corrosion resistance, having stable and excellent mechanical properties for a long time, in particular wear resistance and being excellent in durability, even in a case where it is applied to particularly a load, for example where it is exposed to a dry or a high humidity atmosphere, or in an aqueous medium at an elevated temperature of about 60 to 150° C.
Another object of the present invention is to provide parts that require strength and wear resistance, using the above-mentioned zirconia based sintered bodies, and the parts include for example mill parts for grinding or dispersing apparatuses, wear resistant machine elements, in particular bearings or the like.
Disclosure of Invention
The present inventors have eagerly studied taking the above-mentioned present status into account, and consequently found that lowering of mechanical properties such as strength or wear resistance of zirconia based sintered bodies at an elevated temperature of about 60 to 150° C. in a dry or a high humidity atmosphere, or in an aqueous medium is different from changes in thermal stability at a temperature of 200 to 300° C. in their causes.
That is, thermal degradation at a temperature of 200 to 300° C. results from a transformation from metastable tetragonal zirconia to stable monoclinic zirconia, and it is known that the thermal degradation can be inhibited by incorporating an increased amount of stabilizer, such as Y2O3 or CeO2.
On the other hand, it is assumed that lowering of strength and mechanical properties on exposure to a dry or a high humidity atmosphere, or an aqueous medium at an elevated temperature of about 60 to 150° C. results from that glass phases unhomogeneously present at the grain boundary or the grain boundary face, or amorphous phases Y2O3 and the like segregated at the grain boundary undergo stress corrosion. In the interim, SiO2 is segregated near ZrO2 crystal grain boundary and exerts an effect inhibiting thermal degradation, but it becomes clear that if impurities such as an alkali metal or an alkaline earth metal are present together, glassy phases are formed at ZrO2 crystal grain boundary and thereby resistance against stress corrosion is lowered. From this fact, the present inventors found that it is important to control not only an amount of SiO2 to be added but also an amount of impurities such as an alkali metal or an alkaline earth metal based on that of SiO2 in order to improve physical properties of zirconia sintered bodies.
In addition, the present inventors found also that in order to impart excellent durability to a zirconia sintered body, it is important for SiO2 at ZrO2 crystal grain boundary to be segregated homogeneously in a suitable amount, and it is also important to control an amount of SiO2 to be added depending on its crystal grain size as grain boundary surface areas are different each other depending on the crystal grain size.
The present inventors found that can be obtained zirconia based sintered bodies inhibiting grain boundary corrosion at an elevated temperature of about 60 to 150° C. in a dry or a high humidity atmosphere, or in an aqueous medium, and excellent in durability such as wear resistance even under load, by controlling a molar ratio of Y2O3/ZrO2, an amount of a third component including Al2O3 and SiO2, crystal grain diameter, bulk density, and a relation between the amount of SiO2 and average crystal grain diameter, and a relation between the amount of SiO2 and the amount of impurities, for example alkali metals or alkaline earth metals such as Na2O, K2O or CaO, or Fe2O3 or TiO2, etc., in a specific range, and the present inventors completed the present invention.
That is, a first aspect of the present invention relates to a zirconia based sintered body excellent in durability, characterized in that in Y2O3-toughened zirconia based sintered body,                (a) ZrO2 contained therein is mainly composed of tetragonal ZrO2, and the sintered body contains Y2O3 as a secondary component and further SiO2 and Al2O3 as a third component;        (b) the sintered body has a molar ratio of Y2O3/ZrO2 in the range of 1.5/98.5 to 4/96;        (c) the sintered body has a SiO2 content of 0.05 to 2.5 wt %;        (d) the sintered body has a Al2O3 content of 0.05 to 3.0 wt %;        (e) the sintered body has an average crystal grain diameter of 0.7 μm or less;        (f) SiO2 (wt %)×average crystal grain diameter (μm) takes a value ranging from 0.03 to 0.5;        (g) the sintered body contains at least one oxide of Na2O, K2O, CaO and Fe2O3 as an impurity and a weight ratio of SiO2/(total amount of the impurity) is 5 or more; and        (h) the sintered body has a bulk density of 5.70 g/cm3 or more.        
A second aspect of the present invention relates to the zirconia based sintered body, wherein further contains TiO2 as an impurity and the content of TiO2 is 0.3 wt % or less.
A third aspect of the present invention relates to a parts that requires high strength and wear resistance, characterized in that a working face part or whole of the parts is made of the zirconia based sintered body of the first or second aspect of the present invention.
In the interim, in the present specification and claims, a zirconia based sintered body “excellent in durability” means that the sintered body shows little thermal degradation, or little lowering in strength or change in dimension due to transformation, even when it is exposed to a dry or a high humidity atmosphere, or an aqueous medium at an elevated temperature of about 60 to 150° C. for a long time, and that the sintered body shows little thermal degradation due to transformation from tetragonal zirconia to monoclinic zirconia.
In addition, “a parts that requires high strength and wear resistance” includes parts that are exposed to a high temperature in a state where load is applied on the working face part or whole of the parts, for example mill parts for grinding or dispersing apparatuses, wear resistant machine elements, in particular bearings or element components thereof, or the like.
Further, “mill parts for grinding or dispersing apparatuses” are a general term meaning parts in equipments, jigs or mechanical apparatuses for grinding, dispersing or pulverizing a solid under dry or wet condition, in which the parts contacts the solid, and include grinding and dispersion media. The “grinding or dispersing apparatuses” include concretely grinding and dispersing apparatuses, mixers, granulators, particle size selectors, agitators, dryers, transporting apparatuses and the like.
In addition, bearings include several shaped ones, such as rolling bearings, journal bearings and the like to which the present invention is not limited. The “element parts thereof” include rolling elements used in bearings, and holding parts therefor, or the like.
The followings will explain each requisite that zirconia based sintered bodies excellent in durability of the present invention should fulfil.
(a) As to that the Sintered Bodies are Y2O3-Toughened Zirconia Based Sintered Bodies Mainly Composed of Tetragonal ZrO2 Crystal Phase
If monoclinic zirconia is contained in zirconia based sintered bodies, fine cracks occur near the periphery of the crystal. When the sintered body is under any stress, micro-fracture occurs from the fine cracks and thereby leading to a lowering in resistance against wear, impact, crush or the like. Therefore, the sintered bodies are not preferable as they lead to a reduction of life in mill parts for grinding or dispersing apparatuses, or rolling bearings that are made of the sintered bodies. In addition, the sintered bodies are not preferable as durability is lowered when they are under load in a dry or a high humidity atmosphere, or in an aqueous medium, at an elevated temperature of about 60 to 150° C.
On the other hand, if cubic zirconia is contained in sintered bodies, crystal grain diameter becomes large. Thus, the sintered bodies are not preferable as durability is lowered by not only causing a lowering in mechanical properties but also inducing easily a segregation of Y2O3 near crystal grain boundary.
In the interim, in the present invention, the presence or absence and the content of monoclinic zirconia (M) as zirconia crystal phase, and the contents of tetragonal zirconia (T) and cubic zirconia (C) are determined by X-ray diffraction according to the following process. That is, the surface of a sintered body and an article manufactured from the sintered body is transformed from tetragonal zirconia to monoclinic zirconia due to stress-induced phase transformation, and the real crystal phase thereof can not be identified. Thus, the surface is polished until it becomes a mirror surface, and measured by X-ray diffraction in a range of diffraction angle of 27 to 34 degrees. Thereafter, the presence or absence and the content of monoclinic zirconia are determined and calculated from the following equation:Content of monoclinic zirconia (M)(vol %)=[IM(111)+IM(11{overscore (1)})]/[IM(111)+IM(11{overscore (1)})+IT+C(111)]×100wherein IM(111) is peak height of diffraction peak (111) of monoclinic zirconia, IM(11{overscore (1)}) is peak height of diffraction peak (11{overscore (1)}) of monoclinic zirconia, and IT+C(111) is peak height of a diffraction peak of tetragonal and cubic zirconia.
In addition, the contents of tetragonal zirconia and cubic zirconia are measured by X-ray diffraction in a range of diffraction angle of 70 to 77 degrees, and calculated from the following equation similarly to the process by which the presence or absence of monoclinic zirconia is confirmed:Content of cubic zirconia (C)(vol %)=[IT+C(111)/[IM(111)+IM(11{overscore (1)})+IT+C(111)]]×[IC(400)/[IC(400)+IT(400)+IT(004)]]×100Content of tetragonal zirconia (T)(vol %)=100−M−Cwherein IM(111) is peak height of diffraction peak (111) of monoclinic zirconia, IM(11{overscore (1)}) is peak height of diffraction peak (11{overscore (1)}) of monoclinic zirconia, IT+C(111) is peak height of diffraction peak (111) of tetragonal and cubic zirconia, IC(400) is peak height of diffraction peak (400) of cubic zirconia, and IT(400) and IT(004) are peak heights of diffraction peaks (400) and (004) of tetragonal zirconia, respectively.
Further, in the present invention, “zirconia mainly composed of tetragonal crystal” means that the incorporation of cubic zirconia and monoclinic zirconia is allowed up to 10 vol % and 5 vol %, respectively, in a case where the contents are determined by the above-mentioned X-ray diffraction.
(b) As to that the Zirconia Sintered Body of the Present Invention has a Molar Ratio of Y2O3/ZrO2 in the Range of 1.5/98.5 to 4/96
It is necessary to set Y2O3/ZrO2 molar ratio in the zirconia sintered body of the present invention in the range of 1.5/98.5 to 4/96, preferably 2/98 to 3.5/96.5.
Generally, the sintered body may contain HfO2 that may be contaminated in ZrO2 as a raw material in a little amount. In this case, the total amount of ZrO2 and HfO2 is regarded as the amount of ZrO2.
If Y2O3/ZrO2 molar ratio is less than 1.5/98.5, the content of monoclinic ZrO2 in the zirconia sintered body is increased, and thereby cracks occur in the sintered body, and further the cracks become larger under load for a long time that is a usage condition of mill parts for grinding or dispersing apparatuses or wear resistant machine elements such as bearings to which the sintered bodies is applied, and the sintered body results in fracture or chip. Consequently, the sintered body is not preferable as a lowing in durability is induced. On the other hand., if Y2O3/ZrO2 molar ratio is more than 4/96, the content of tetragonal ZrO2 is decreased and the content of cubic ZrO2 is increased. Such a sintered body is not preferable as mechanical properties are lowered.
In the interim, up to 30 mol % of the amount of added Y2O3 may be replaced with one or more rare earth oxides other than Y2O3 Such rare earth oxides include CeO2, Nd2O3, Yb2O3, Dy2O3 and the like that are low-cost and preferable.
(c) As to that the Sintered Body has a SiO2 Content of 0.05 to 2.5 wt %
It is necessary for the zirconia sintered body of the present invention to contain SiO2 of 0.05 to 2.5 wt %, preferably 0.05 to 2.0 wt %. SiO2 is segregated near ZrO2 crystal grain boundary and therefore present in ZrO2 crystal grain boundary. The SiO2 has effects that stress corrosion is inhibited and further the bonding of ZrO2 crystal grain boundary is enhanced, and thus SiO2 makes possible to improve durability in case where load is applied. If the content of SiO2 is more than 2.5 wt %, amorphous phase and glassy phase are formed much at ZrO2 crystal grain boundary, the bonding of ZrO2 crystal grain boundary is lowered, stress corrosion occurs easily and the effect of stress-induced phase transformation from tetragonal to monoclinic phase is lowered. Consequently, it is not preferable as a lowering in mechanical properties and durability is led. If SiO2 is less than 0.05 wt %, SiO2 is not partly present in ZrO2 crystal grain boundary, and thereby stress corrosion can not be inhibited and durability is lost, therefore is not preferable.
(d) As to that the Sintered Body has a Al2O3 Content of 0.05 to 3.0 wt %
It is necessary for the zirconia sintered body of the present invention to contain Al2O3 of 0.05 to 3.0 wt %, preferably 0.05 to 2.0 wt %. Al2O3 is not only present at ZrO2 crystal grain boundary as Al2O3 crystal grain but also segregated at ZrO2 crystal grain boundary and extremely near grain boundary. The addition of Al2O3 has not only an effect on improvement of sinterability and homogeneity of fine structure but also an effect on inhibition of lowering in properties due to exposure to a dry or a high humidity atmosphere or an aqueous medium at an elevated temperature of about 60 to 150° C. In addition, as Al2O3 has an effect on strengthening of ZrO2 crystal grain boundary, it confers excellent mechanical properties, such as high-impact strength to the sintered body. If the content of Al2O3 is less than 0.05 wt %, the effect by the addition of Al2O3 is not exerted, while if the content of Al2O3 is more than 3.0 wt %, many Al2O3 crystal grains are present at ZrO2 crystal grain boundary, thereby durability is lowered and therefore it is not preferable.
(e) As to that the Sintered Body has an Average Crystal Grain Diameter of 0.7 μm or Less
It is necessary for the zirconia sintered body of the present invention to have an average crystal grain diameter of 0.7 μm or less, more preferably 0.5 μm or less. Although it is preferable to have smaller crystal grain diameter as mechanical properties, such as wear resistance are improved, the crystal grain diameter of about 0.1 μm is lower limit in the present pulverizing technique.
If the average crystal grain diameter is more than 0.7 μm, not only durability but also mechanical properties, such as wear resistance are lowered, and it is not preferable.
In the interim, the average crystal grain diameter is determined as follows. The surface of a sintered body is polished until it becomes a mirror surface, and then the polished body is exposed to thermal etching or chemical etching, and thereafter observed at ten points thereon with a scanning electron microscope according to the intercept method. The average of the resulting ten values is the average crystal grain diameter and is calculated from the following equation:
 D=1.5×L/n
wherein D is an average crystal-grain diameter (μm), n is the number of crystal grains per length L, and L is a measured length (μm).
(f) As to that SiO2 (wt %)×Average Crystal Grain Diameter (μm) Takes a Value Ranging from 0.03 to 0.5
It is necessary for the zirconia sintered body of the present invention to have a value of SiO2 (wt %)×average crystal grain diameter (μm) ranging from 0.03 to 0.5, preferably from 0.03 to 0.3.
SiO2 is segregated near ZrO2 crystal grain boundary, and thus inhibits stress corrosion and improves durability. However, grain boundary surface areas are different each other depending on the crystal particle size. Therefore, it is necessary to control an amount of SiO2 to be added depending on its crystal particle size in order to make SiO2 present homogeneously at crystal grain boundary.
If the value of SiO2 (wt %)×average crystal grain diameter (μm) is more than 0.5, SiO2 present near ZrO2 crystal grain boundary becomes in excess, thereby resulting in amorphous phase and glass phase much at grain boundary, a lowering in the bonding strength of ZrO2 crystal grain boundary, an easy progress of stress corrosion and a lowering of the effect by stress-induced phase transformation from tetragonal phase to monoclinic phase. Therefore, it is not preferable as mechanical properties, wear resistance and durability are lowered. If the value of SiO2 (wt %)×average crystal grain diameter (μm) is less than 0.03, SiO2 is not partly present in ZrO2 crystal grain boundary, and thereby stress corrosion can not be inhibited and durability is lost, therefore it is not preferable.
(g) As to that the Sintered Body Contains at Least One Oxide of Na2O, K2O, CaO and Fe2O3 as an Impurity and a Weight Ratio of SiO2/(Total Amount of the Impurity) is 5 or More
The weight ratio of SiO)(total amount of the impurity) will be expressed also in the weight ratio of SiO2/(total amount of Na2O, K2O, CaO and Fe2O3), and it is necessary to have 5 or more, preferably 10 or more. If the weight ratio of SiO2/(total amount of Na2O, K2O, CaO and Fe2O3) is less than 5, glassy phase of SiO2 with Na2O, K2O, CaO and Fe2O3 is formed much at grain boundary, thereby resulting in corrosion. Consequently, the bonding strength of ZrO2 crystal grain boundary is lowered, stress-induced phase transformation from tetragonal crystal to monoclinic phase is lowered, and thus it leads to a lowering in mechanical properties and durability, and therefore is not preferable.
The impurities, such as Na2O, K2O, CaO and Fe2O3, etc. are not intentionally mixed, and it is hard to cause glassy phase or amorphous phase and desired as the content becomes low. However, the lowest total amount of the impurity is about 0.008 wt % in the present technique.
(h) As to that the Sintered Body of the Present Invention has a Bulk Density of 5.70 g/cm3 or More
It is necessary for the zirconia sintered body of the present invention to have a bulk density of 5.70 g/cm3 or more, preferably 5.80 g/cm3 or more.
If the bulk density is less than 5.70 g/cm3, the sintered body is inferior in resistance against external stress such as friction or impact and at the same time, the sintered body has a lowered resistance against corrosion at a high temperature of about 60 to 150° C. or in an aqueous medium or a high humidity atmosphere at the above-mentioned high temperature, and therefore it is not preferable.
The bulk density is lowered a little as the content of SiO2 becomes high. However, the theoretical density is 6.1 g/cm3 in a range of the composition of the present invention, and therefore the value is essentially the upper limit.
(i) As to that the Zirconia Sintered Body of the Present Invention has a Content of TiO2 of 0.3 wt % or Less
The zirconia sintered body of the present invention has a content of TiO2 of 0.3 wt % or less, preferably 0.2 wt %, more preferably 0.1 wt %.
If the content of TiO2 is more than 0.3 wt %, TiO2 is solid-dissolved in ZrO2, the growth of crystal grain diameter is promoted and the crystal grain diameter becomes large, and thus wear resistance and durability are lowered and it is not preferable. TiO2 is impurity and the lower the content thereof is, the more desirable it is. However, the lowest content of TiO2 impurity is about 0.004 wt % in the present technique.
The zirconia based sintered body excellent in durability of the present invention can be produced by several methods. An example among them is explained below, but the present invention is not limited thereto.
The present invention requires a usage of zirconia powders purified by a liquid-phase method. That is, an aqueous solution of a zirconium compound (for example, zirconium oxychloride) is mixed with an aqueous solution of an yttrium compound (for example, yttrium chloride) homogeneously in such a manner that the molar ratio of ZrO2 and Y2O3 falls in the prescribed one, the mixture is hydrolyzed to give hydrates, the resulting hydrates are dehydrated, dried and calcined at a temperature of 400 to 1250° C. to produce zirconia powders containing a small quantity of impurity other than Y2O3, Al2O3 or SiO2.
The components other than Y2O3 may be added as an aqueous solution of its salt in the prescribed amount into a mixture of an aqueous solution of zirconium compound and an aqueous solution of yttrium compound, or added in a state of hydroxide, carbonate, oxide or the like when calcined powders are ground and dispersed as described below.
Molding powders are produced by grinding and dispersing the resulting calcined powders in wet, optionally adding a known molding auxiliary (e.g., wax emulsion, PVA, an acrylic resin or the like) and drying with a known method, such as spray dry or the like. The resulting molding powders are required to have an average particle size of 0.5 μm or less, more preferably 0.4 μm or less. When the average particle size is more than 0.5 μm, a lowering in sinterability and a lowering in durability and mechanical properties of the sintered body are led, and therefore it is not preferable.
From the resulting molding powders, sintered body in a shape suitable for a desired use can be obtained with a known method, such as press molding, rubber press molding or the like. However, a more preferable method is a method of molding in an organic solvent containing water with a molding auxiliary such as a soluble polymer or water in wet or in a liquid.
Next, the resulting molded products are sintered at a temperature of 1150 to 1550° C., preferably 1150 to 1400° C. to obtain sintered bodies. Further, the sintered bodies are processed in a desired shape to produce parts that require durability and/or wear resistance, for example mill parts for grinding and/or dispersing apparatuses, or bearings. The above-mentioned molded products may be optionally exposed to HIP treatment and improved in resistance against friction, impact, crash and the like, in mechanical properties and further in durability. The HIP treatment is preferably carried out in an inert atmosphere such as Ar, or N2 or O2 atmosphere at a temperature of 1100 to 1400° C.